1. Field of the Invention
The present invention relates to an image processing apparatus suitable for electronic file, facsimile machine, reader, digital copying machine or the like.
2. Description of the Prior Art
In a gray level (half tone) processing system of a prior art image processing apparatus of this type, a systematic dither method in which an image signal level of each pixel is compared with a respective threshold level of a threshold matrix and black is detected when the image signal level is higher than the threshold level and white is detected when the image signal level is lower than the threshold level, to reproduce a gray level, has been usually utilized.
In this method, however, resolution power is much lower than that in a simple binary recording method using a fixed threshold, and image quality of a high resolution area such as character areas is degraded.
In order to resolve the above problem, a method has been proposed in Japanese Patent Application Laid-Open No. 3374/1983 in which an image is divided into a plurality of blocks and a difference between a maximum pixel density level and a minimum pixel density level in each block is used to determine whether the block is a gray level image area which requires high tonality reproduction, such as a photograph or picture, or a binary image (line image) area which requires high resolution, such as characters or line image.
In this method, however, circuits for determining the maximum density and the minimum density are complex and operation time therefor is required. In addition, a large capacity image buffer memory for storing a gray level image is required.
In the prior art image processing apparatus of this type, for the block determined to be the line image area, the original image information is binarized by a predetermined fixed threshold to produce a binary image, and for the block determined to be the gray level area, the original image information is dither-processed to produce a binary image.
FIG. 1 shows an example of original image information in which a character image having pixel values of 180 appears on the right of a gray level image having pixel values of 50 to 60. The image shown in FIG. 1 is divided into blocks each consisting of four pixels vertically and eight pixels horizontally, and each block is checked to determine whether it is a gray level area or a character area (line image area) based on the difference between the maximum density and the minimum density in the block (see Japanese Patent Application Laid-Open No. 3374/1984). Thus, the left block is determined as the gray level area (H) and the right block is determined as the character block (B), as shown in FIG. 2.
FIG. 3 shows an example of a systematic dither matrix having 32 thresholds. FIG. 4A shows an image produced by binarizing the original image of FIG. 1 by a fixed threshold 128, and FIG. 4B shows an image produced by binarizing the original image of FIG. 1 by the systematic dither matrix of FIG. 3.
In FIG. 4C, the images of FIGS. 4A and 4B are combined for each block in accordance with the decision shown in FIG. 2. Since the left block of FIG. 1 was determined to be a gray level area, the left block of FIG. 4B is selected for the left block of FIG. 4C, and since the right block of FIG. 1 was determined to be a character block, the right block of FIG. 4A is selected for the right block of FIG. 4C.
As a result of the area determination, the degradation in the character area (see black area 45 in FIG. 4B) which appeared in the systematic dither processing is significantly improved, as seen from black area 48 in FIG. 4C, but the gray level information around the characters (see area 44 in FIG. 4B) is dropped in FIG. 4C. As a result, peripheries of characters appear white and discontinuity with the background appears Thus, a very unnatural image is reproduced.